Search

Obtaining reliable petrochronological and geochemical data from metamict zircon may be challenging. Metamict zircon and crystalline apatite from the Meiwu granodiorite and its microgranular enclaves from the Paleo-Tethys belt are examined to constrain their crystallization ages and the genetic mechanism of related skarn mineralization. The metamict zircon yields highly disturbed 206Pb/238U dates. Transmission electron microscopy shows that radiation damage forms nanoscale-banded damaged zones, leading to spurious dates. The coexisting apatite has not accumulated radiation damage, and apatite crystals from the granodiorite and its enclaves yield reasonably precise LA-ICPMS U–Pb Tera–Wasserburg concordia lower intercept dates of 240.2 ± 3.8 and 239.9 ± 4.0 Ma (2σ), with MSWDs of 1.0 and 2.1. Considering the fast cooling of the granite, the U–Pb dates effectively represent crystallization ages for these rocks. Compositional analysis shows that there are no Ce anomalies in apatite in either the granodiorite or enclave, indicating low oxygen fugacities. Apatite crystals from enclaves have weaker negative Eu anomalies, higher Sr, and lower HREE and Y contents than those in granodiorite. The compositions confirm enclaves as products of water-rich melts, resulting in amphibole fractionation and suppression of plagioclase crystallization. The hydrous magma induced production of hydrothermal-fluids that mobilized metals dispersed in dry magma and concentrated them into mineralization traps, which contributed to the formation of widespread skarns in Paleo-Tethys belt. This study demonstrates that apatite is effective in tracing the evolution of magmatic systems containing metamict zircon.

Alternating layers of pelitic and basic bands with occasional semi-pelitic band, millimeter to centimeter in width, occur between the epidote–amphibolite (metagabbro) and the pelitic schist in the epidote–amphibolite facies region of the Sanbagawa metamorphic belt, central Shikoku. The whole-rock major, trace, and rare earth element compositions of the semi-pelitic band are intermediate between those of the basic and pelitic bands. The peak metamorphic conditions were estimated at 1.0–1.2 GPa/600–630 °C for the mineral assemblage of the pelitic and semi-pelitic bands. The evolution of the CO2-rich fluid [X(CO2) = CO2/(CO2 + H2O)] at the lithologic boundary between the epidote–amphibolite and the pelitic schist, during the Sanbagawa prograde metamorphism, is discussed in the context of a titanite, rutile, calcite, dolomite, and quartz assemblage. The X(CO2) of the semi-pelitic band and basic and the pelitic bands increased during prograde metamorphism from the stability field of titanite to those of rutile + dolomite + amphibole + quartz and rutile + calcite + amphibole + quartz, respectively. The X(CO2) values of the metamorphic fluid at the peak metamorphic stage estimated by the matrix assemblages were higher in the order of the pelitic schist and epidote–amphibolite (less than 0.12–0.23), basic and pelitic bands (0.23–0.38), and semi-pelitic band (0.38–0.57), suggesting variations in the fluid compositions on a millimeter to centimeter scale. The CO2-rich fluid in the alternating layers, especially in the semi-pelitic band, was probably generated by a redox reaction between Fe3+-bearing silicate phases, such as amphibole and epidote in the basic band and carbonaceous material in the pelitic band. This reaction is thought to have been triggered by the chemical and/or mechanical mixing of these two bands during prograde metamorphism, resulting in the formation of the semi-pelitic band.

In Late Proterozoic mafic granulites of the Sandmata Complex in Rajasthan, northwestern India, multilayer corona textures were formed along the interface between orthopyroxene and plagioclase. We examined the metamorphic conditions and processes of formation of these coronae, which provide an insight into the interplay between steady state and sequential diffusion-controlled mineral growth mechanisms. The individual corona–symplectite layers consist of clinopyroxene + quartz|garnet + clinopyroxene|garnet + quartz|K-feldspar, from the inner to the outer margins of the coronae. The single-value decomposition models suggest that the multilayered coronae were formed in a locally closed system, via sequential diffusion of Mg, Fe and Ca into the reaction zone, which has acted as the main driving force for the growth of corona textures. The relict orthopyroxene with exsolved clinopyroxene yields primary crystallization conditions of 8.4 ± 1.5 kbar and ~ 1100–1000 °C. Clinopyroxene in the innermost corona layer grew at ~ 9 kbar and 850–800 °C, whereas clinopyroxene + garnet grew outward at ~ 8 kbar and 700–600 °C. Subsequent hydrous retrogression (~ 6 kbar and 600–550 °C) resulted in the development of rimward zoning in garnet and the growth of amphibole. On combining textural relations and the above conventional P–T estimates, a near-isobaric cooling P–T path was reconstructed using phase equilibria modeling. Further, the near-isobaric cooling path is consistent with the magmatic underplating hypothesis in the Sandmata Complex, where the intrusion of magmatic bodies (i.e., Gyangarh–Asind igneous complex and Anjana granite) favored the development of granulite facies assemblage in norite and gabbronorite protoliths.

The Ditrau Complex of the Carpathian Mountains in Romania is a Mesozoic igneous complex (~ 200–230 Ma) generated in a continental rift environment. Felsic rocks of the Ditrau Complex consist of nepheline syenite, syenite, quartz syenite, quartz monzonite, monzonite and granite. The Ditrau rocks have mantle-like ɛNd values that range from + 0.8 to + 5.5 ‰. High-temperature equilibrium O-isotope fractionations between minerals are generally preserved, although some subsolidus O-isotope re-equilibration occurred. Magma δ18O values estimated from quartz, feldspar and amphibole (5.7–11.7‰) are higher than those estimated from zircon. We suggest that this difference results from continuous crustal contamination, with zircon recording the early, high-temperature δ18O values, and quartz and the other silicate δ18O values, reflecting a combination of subsequent crustal contamination and deuteric alteration. Negative correlations between calculated magma δ18O values and Na2O and Al2O3 content and eNd are consistent with the spectrum of felsic rocks from nepheline syenite to granite resulting from an increase crustal input. Both O- and Nd-isotope compositions are consistent with a dominantly mantle origin of hornblendites, diorites and nepheline syenites. The Nd- and O-isotope composition of the silica-oversaturated rocks can be explained by the assimilation of 20–60% upper crustal melts into the re-injected mafic alkaline parent magma to generate the Ditrau syenites, quartz syenites, quartz monzonites and granites.

The stability field and composition of immiscible sulfate melts in equilibrium with silicate magmas has been determined using experiments over a range of crustal pressures, allowing an assessment of their possible role in transporting sulfur into the sub-volcanic arc and porphyry copper deposit systems. Experimental starting materials were based on natural trachy- andesite and trachydacite compositions, with 3.5–7 wt% H2O and 3.5–5.5 wt% sulfur added to produce large, analyzable amounts of sulfate phases. Conditions ranged over 800–1200 °C, 0.2-1GPa and ƒO2 > NNO + 2.5. Sulfate melts formed at temperatures above 1000 °C at 0.75 and 1 GPa and above 900 °C at 0.2 GPa, suggesting some pressure dependence on their stability. At temperatures below 1100 °C sulfate melts and anhydrite crystals commonly coexist. Sulfate melts quenched to an intergrowth that was difficult to prepare for analysis. However, the composition was approximated by EPMA and further constrained by mass balance calculations. Sulfate melts were dominated by CaO and SO3, but also contained, in order of decreasing abundance, Na2O, K2O, MgO, FeO, Cl and P2O5. Chlorine showed a particular preference for the sulfate melt relative to the coexisting silicate melt, and calculated partition coefficients for sulfate/silicate melts were 5–13 at 1200 °C, 0.75–1 GPa. Experimental data show that, in the absence of an exsolved, hydrous fluid phase, sulfate melts can form in natural arc magmas at near-liquidus temperatures ≥ 1000 °C, assuming that magmas are oxidized and contain sufficient sulfur (> 2000–3000 ppm S). These results suggest that sulfate melt could be an important component in transporting sulfur as well as chlorine to shallow levels in the crust for hydrous magmas under a specific range of conditions. Both the non-quenchable and water-soluble nature of sulfate melts (and anhydrite) make them difficult to identify, unless trapped as mineral inclusions similar to the “wormy anhydrite” trapped in high-temperature amphiboles from Yanacocha, Peru.

We measured the electrical conductivity of tremolite over a range of pressures (1.0, 1.5, and 2.0 GPa) and temperatures (648–1373 K). At temperatures lower than 1173 K, the electrical conductivity of tremolite was ~ 0.001 S/m, but once the dehydroxylation reaction took place at 1173 K, we observed a significant increase in the electrical conductivity. The electrical conductivity continued to increase until it reached its maximum value of ~ 1 S/m at a temperature of 1373 K, which we attributed to dehydroxylation in the tremolite samples. The electrical behavior was altered by the physical breakdown of the tremolite structure, rather than the oxidation process. The decomposition of tremolite resulted in the creation of diopside, enstatite, and quartz. To explain some anomalous conductivity measurements, we propose that iron-free amphibole experiences a dehydroxylation mechanism that differs from the dehydroxylation mechanism observed in iron-bearing amphiboles. Our conductivity analyses indicate that the observed increase in the electrical conductivity in deep subduction regions (at depths of ~ 180 km) is driven by the dehydroxylation of tremolite and by the mixing of lherzolite and tremolite in the upper mantle. Using our data and previously published temperature gradient data, we were able to extrapolate reasonable electrical conductivity values for the surface and interior of Venus.

This study presents a new petrological–geochemical data set for the Monte Capio and Alpe Cevia mafic–ultramafic sequences, which are exposed in the deepest levels of the Ivrea–Verbano Zone. These sequences are composed of a peridotite core, with dunite in the center, mantled by minor orthopyroxene-dominated pyroxenites and subordinate hornblende gabbronorites. Amphibole is ubiquitous in the peridotites and the pyroxenites (≤ 15 vol % and 10–40 vol %, respectively), and the peridotite–pyroxenite associations are frequently crosscut by amphibole-rich (45–90 vol %) veins/dykes showing sinuous-to-sharp planar boundaries towards host rocks. The whole-rock Mg# [100 × Mg/(Mg + Fetot2+)] decreases from the peridotites to the pyroxenites and the crosscutting amphibole-rich dykes (84–81, 80–77, and 73–66, respectively), consistently with the Mg# variations shown by included orthopyroxene, clinopyroxene, and amphibole. Olivine has relatively low forsterite and NiO amounts (84–78 mol % and ≤ 0.14 wt%), and spinel is characterized by low Cr# [100 × Cr/(Cr + Al)] of 7–24. The anorthite content of plagioclase varies from 91 to 88 mol% in plagioclase-bearing pyroxenites to 91–75 mol% in amphibole-rich dykes. The chondrite-normalized REE patterns of amphibole from peridotites and pyroxenites show nearly flat MREE–HREE, no evident Eu anomaly, and LREE that are slightly depleted to slightly enriched with respect to MREE. Amphibole from the amphibole-rich veins/dykes exhibits slight LREE depletion. Whole-rock and amphibole separates show substantial variations in initial Nd–Sr isotopic compositions (e.g., whole-rock eNd calculated at 290 Ma ranges from − 0.3 to − 4.7), irrespective of the rock-type and of incompatible element amphibole compositions. We propose that the Monte Capio–Alpe Cevia dunites formed by cooling of magma lenses that intruded the lowermost continental crust of the Ivrea–Verbano Zone. The chemically evolved signature of the dunites documents earlier crystallization of chemically primitive dunites at lower levels, or olivine fractionation within the dunites during melt ascent. Associated pyroxene-bearing peridotites show a magmatic evolution ruled by reaction of a melt-poor crystal mush with migrating melts relatively rich in SiO2 and H2O, which developed orthopyroxene and amphibole at the expenses of olivine ± clinopyroxene. These migrating melts may be reconciled with those feeding the crosscutting amphibole-rich veins/dykes, whose compositions suggest formation by chemically evolved H2O-rich basalts with an arc-type incompatible trace-element fingerprint. Unraveling the origin of the Monte Capio–Alpe Cevia pyroxenites is hampered by the complex open-system magmatic evolution, which also included assimilation of material released by basement metasediments and/or involvement of primary melt batches with different compositions.

Quantifying the storage conditions and evolution of different magmatic components within sub-volcanic plumbing systems is key to our understanding of igneous processes and products. Whereas erupted magmas represent a portion of the eruptible volcanic system, plutonic xenoliths provide a complementary record of the mushy roots of the plumbing system that cannot be mobilised easily to form lavas and consequently offer a unique record of magma diversity within the sub-volcanic plumbing system. Here, we present a detailed petrological and geochemical study of erupted plutonic xenoliths from the island of Sint Eustatius (Statia), in the northern Lesser Antilles volcanic arc. The plutonic xenoliths are predominantly gabbroic, but vary in texture, mineral assemblage and crystallisation sequence. We report major, trace and volatile (H2O and CO2) concentrations of xenolith-hosted melt inclusions (MIs) and interstitial glass. The MIs have a very large range in major element (49–78 wt% SiO2 and 0.1–6.1 wt% MgO) and trace element concentration (72–377 ppm Sr, 32–686 ppm Ba, 39–211 ppm Zr). Their chemistry varies systematically with host phase and sample type. Significantly, it shows that (1) plutonic xenoliths record a complete differentiation sequence from basalt to rhyolite (2) apatite, but not zircon, saturation was reached during crystallisation, (3) amphibole breakdown reactions play a role in the genesis of shallow gabbronorite assemblages, and (4) mixing between crystal cargos and multiple discrete bodies occurred. Residual melt volatile contents are high (≤ 9.1 wt% H2O and ≤ 1350 ppm CO2), returning volatile saturation pressures of 0–426 MPa. Multiple reaction geobarometry and experimental comparisons indicate that equilibration took place in the upper-middle crust (0–15 km). We infer that the Statia plutonic xenoliths represent portions of a large heterogeneous crystal mush within which a great diversity of melts was stored and mixed prior to eruption. Our data show that compositional variations in magmatic plumbing systems exceed those observed in volcanic products, a likely consequence of the blending that occurs prior to and during eruption. Electronic supplementary material The online version of this article (10.1007/s00410-019-1576-4) contains supplementary material, which is available to authorized users.

International audience; A group of ultramafic xenoliths hosted in Cenozoic hypabyssal rocks from Harrow Peaks (northern Victoria Land, Antarctica) show textural and geochemical features far removed from anything previously observed in mantle xenoliths of this region and elsewhere in Antarctica. They consist of spinel-bearing lherzolites and harzburgites, characterised by a predominant equigranular texture with orthopyroxene modal contents remarkably higher in lherzolites (18-26 volume%) with respect to the harzburgite (13 vol%), one orthopyroxenite, and three composite xenoliths. The latter are formed by an olivine-dominant assemblage (olivine > 70%) crosscut by large monomineralic (amphibole or clinopyroxene) or bimineralic (amphibole + clino-pyroxene) veins. No significant correlation was observed between the lithology and the Fo content (90.21-82.81) of olivine, suggesting that these rocks could be derived from a cumulus process. The presence of the orthopyroxenite suggests that the inferred melt/s from which they stemmed was close (or even above) to silica saturation. Based on major and trace-element mineral/melt and mineral/mineral equilibrium modelling, these rocks were formed by progressive extraction of olivine from a high magnesium (Mg = 72)-high temperature (~ 1300 °C) melt following a very short fractionation line. Thermobaro-metric results indicate the stationing of Harrow Peaks cumulates in the P field of 1.3 ± 0.2 (dunites)-0.5 ± 0.2 (orthopy-roxenite) GPa. These values well match the crust/mantle boundary (Moho) of the region. The combined geochemical and petrological data suggest that Harrow Peaks melts could be related to the initial stage of the Jurassic Ferrar magmatism, whose deep cumulates were subsequently affected by the Cenozoic alkaline metasomatism, widely detected in the northern Victoria Land lithosphere and responsible for the formation of the late amphibole/amphibole + clinopyroxene veins.

Volcanoes at subduction zones reside above complex magma plumbing systems, where individual magmatic components may originate and interact at a range of pressures. Because whole-rock compositions of subduction zone magmas are the integrated result of processes operating throughout the entire plumbing system, processes such as mixing, homogenisation and magma assembly during shallow storage can overprint the chemical signatures of deeper crustal processes. Whereas melt inclusions provide an effective way to study the uppermost 10–15 km of the plumbing system, challenges remain in understanding magma intrusion, fractionation and hybridisation processes in the middle to lower crust (15–30 km depth), which commonly involves amphibole crystallisation. Here, we present new insights into the mid-crustal plumbing system at Mount St. Helens, USA, using multiple regression methods to calculate trace element partition coefficients for amphibole phenocrysts, and thus infer the trace element compositions of their equilibrium melts. The results indicate vertically distributed crystal fractionation, dominated by amphibole at higher pressures and in intermediate melts, and by plagioclase at lower pressures. Variations in Nb, Zr and REE concentrations at intermediate SiO2 contents suggest repeated scavenging of partially remelted intrusive material in the mid-crust, and mixing with material from geochemically diverse sources. Amphibole is an effective probe for deep crustal magmatism worldwide, and this approach offers a new tool to explore the structure and chemistry of arc magmas, including those forming plutonic or cumulate materials that offer no other constraints on melt composition.

High-temperature decompression experiments demonstrate that crystal textures preserve a record of the style and rate of magmatic ascent. To reinforce this link, we performed a suite of isothermal decompression experiments using starting material from the climactic 1991 Pinatubo eruption. We decompressed experiments from 220 MPa to final, quench pressures of 75 or 30 MPa using continuous decompression rates of 100, 30, 10, 3, 1, and 0.3 MPa h−1. Amphibole, clinopyroxene, and plagioclase crystallized during the experiments, with plagioclase microlites dominating the assemblage. Total microlite number densities range from 107.6±0.4 up to 108.2±0.2 cm−3, with plagioclase accounting for up to 65% of the total number. Plagioclase microlite area increased systematically from 19 ± 8 to 937 ± 487 µm2 with increasing experiment duration. Our textures provide time-integrated records of crystal kinetics. Average nucleation and areal growth rates of plagioclase are highest in the fastest decompressions (~ 107.5 cm−3 h−1 and 10.1 ± 4.1 µm2 h−1, respectively) and more than an order of magnitude lower in the slowest experiments (~ 105.5 cm−3 h−1 and 0.8 ± 0.2 µm2 h−1, respectively). Both nucleation and growth rates are highest at high degrees of disequilibrium. We find that peak supersaturation-dependent instantaneous rates are generally more than an order of magnitude faster than average rates. We use those instantaneous nucleation and growth rates to introduce an iterative model to evaluate the effects of different decompression rates, decompression paths (continuous, single-step or multistep), and the presence of phenocrysts on final crystallinity and microlite size distribution.

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An ≤ 98) and low forsterite (Fo ≤ 84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H2O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H2O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H2O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.

Numerous lamproite dykes are hosted by the Eastern Dharwar Craton, southern India, particularly towards the northwestern margin of the Cuddapah Basin. We present here a comprehensive mineralogical and geochemical (including Sr and Nd isotopic) study on the lamproites from the Vattikod Field, exposed in the vicinity of the well-studied Ramadugu lamproite field. The Vattikod lamproites trend WNW–ESE to NW–SE and reveal effects of low-temperature post-magmatic alteration. The studied lamproites show porphyritic texture with carbonated and serpentinized olivine, diopside, fluorine-rich phlogopite, amphibole, apatite, chromite, allanite, and calcite. The trace-element geochemistry (elevated Sr and HFSE) reveals their mixed affinity to orogenic as well as anorogenic lamproites. Higher fluorine content of the hydrous phases coupled with higher whole-rock K2O highlights the role of metasomatic phlogopite and apatite in the mantle source regions. Trace-element ratios such as Zr/Hf and Ti/Eu reveal carbonate metasomatism of mantle previously enriched by ancient subduction processes. The initial 87Sr/86Sr-isotopic ratios (calculated for an assumed emplacement age of 1350 Ma) vary from 0.7037 to 0.7087 and ɛNd range from − 10.6 to − 9.3, consistent with data on global lamproites and ultrapotassic rocks. We attribute the mixed orogenic–anorogenic character for the lamproites under study to multi-stage metasomatism. We relate the (1) earlier subduction-related enrichment to the Paleoproterozoic amalgamation of the Columbia supercontinent and the (2) second episode of carbonate metasomatism to the Mesoproterozoic rift-related asthenospheric upwelling associated with the Columbia breakup. This study highlights the association of lamproites with supercontinent amalgamation and fragmentation in the Earth history.

Magnetite from hydrothermal ore deposits can contain up to tens of thousands of parts per million (ppm) of elements such as Ti, Si, V, Al, Ca, Mg, Na, which tend to either structurally incorporate into growth and sector zones or form mineral micro- to nano-sized particles. Here, we report micro- to nano-structural and chemical data of hydrothermal magnetite from the Los Colorados iron oxide–apatite deposit in Chile, where magnetite displays both types of trace element incorporation. Three generations of magnetites (X–Z) were identified with concentrations of minor and trace elements that vary significantly: SiO2, from below detection limit (bdl) to 3.1 wt%; Al2O3, 0.3–2.3 wt%; CaO, bdl–0.9 wt%; MgO, 0.02–2.5 wt%; TiO2, 0.1–0.4 wt%; MnO, 0.04–0.2 wt%; Na2O, bdl–0.4 wt%; and K2O, bdl–0.4 wt%. An exception is V2O3, which is remarkably constant, ranging from 0.3 to 0.4 wt%. Six types of crystalline nanoparticles (NPs) were identified by means of transmission electron microscopy in the trace element-rich zones, which are each a few micrometres wide: (1) diopside, (2) clinoenstatite; (3) amphibole, (4) mica, (5) ulvospinel, and (6) Ti-rich magnetite. In addition, Al-rich nanodomains, which contain 2–3 wt% of Al, occur within a single crystal of magnetite. The accumulation of NPs in the trace element-rich zones suggest that they form owing to supersaturation from a hydrothermal fluid, followed by entrapment during continuous growth of the magnetite surface. It is also concluded that mineral NPs promote exsolution of new phases from the mineral host, otherwise preserved as structurally bound trace elements. The presence of abundant mineral NPs in magnetite points to a complex incorporation of trace elements during growth, and provides a cautionary note on the interpretation of micron-scale chemical data of magnetite.

The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo89.4−90.5), enstatite (En89 − 91Fs8.7−9.8Wo0.82−1.13), clinopyroxene, spinel (Cr#Sp = 9.4–13.8), and in some cases amphibole (Mg# = 88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al2O3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al2O3 = 6.07–6.56 wt% in clinopyroxene) and the low CaO/Al2O3 ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.

Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.

Shock-induced melt veins in amphibolites from the Nordlinger Ries often have chemical compositions that are similar to bulk rock (i.e., basaltic), but there are other veins that are confined to chlorite-rich cracks that formed before the impact and these are poor in Ca and Na. Majoritic garnets within the shock veins show a broad chemical variation between three endmembers: (1) $${}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}{\text{Al}}_{2} ({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (normal garnet, Grt), (2) $${}^{\text{VIII}}{{\text{M}^{2+}}_3} {}^{\text{VI}}[{\text{M}}^{2 + } ({\text{Si,Ti}})]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (majorite, Maj), and (3) $${}^{\text{VIII}}({{\text {Na} {\text M}^{2+}}_2}) {}^{\text{VI}}[ ({\text{Si,Ti}}){\text {Al}}]({}^{\text{IV}}{\text{SiO}}_{4} )_{3}$$ (Na-majorite50Grt50), whereby M2+ = Mg2+, Fe2+, Mn2+, Ca2+. In particular, we observed a broad variation in VI(Si,Ti) which ranges from 0.12 to 0.58 cations per formula unit (cpfu). All these majoritic garnets crystallized during shock pressure release at different ultrahigh pressures. Those with high VI(Si,Ti) (0.36–0.58 cpfu) formed at high pressures and temperatures from amphibole-rich melts, while majoritic garnets with lower VI(Si,Ti) of 0.12–0.27 cpfu formed at lower pressures and temperatures from chlorite-rich melts. Furthermore, majoritic garnets with intermediate values of VI(Si,Ti) (0.24–0.39) crystallized from melts with intermediate contents of Ca and Na. To the best of our knowledge the ‘MORB-type’ Ca–Na-rich majoritic garnets with maximum contents of 2.99 wt% Na2O and calculated crystallisation pressures of 16–18 GPa are the most extreme representatives ever found in terrestrial shocked materials. At the Ries, the duration of the initial contact and compression stage at the central location of impact is estimated to only ~ 0.1 s. We used a ~ 200-µm-thick shock-induced vein in a moderately shocked amphibolite to model its pressure–temperature–time (P–T–t) path. The graphic model manifests a peak temperature of ~ 2600 °C for the vein, continuum pressure lasting for ~ 0.02 s, a quench duration of ~ 0.02 s and a shock pulse of ~ 0.038 s. The small difference between the continuum pressure and the pressure of majoritic garnet crystallization underlines the usefulness of applying crystallisation pressures of majoritic garnets from metabasites for calculation of dynamic shock pressures of host rocks. Majoritic garnets of chlorite provenance, however, are not suitable for the determination of continuum pressure since they crystallized relatively late during shock release. An extraordinary glass- and majorite-bearing amphibole fragment in a shock-vein of one amphibolite documents the whole unloading path.

Different processes have been proposed to explain the variety of igneous layering in plutonic rocks. To constrain the mechanisms of emplacement and crystallization of ascending magma batches in shallow plutons, we have studied comb layers and orbicules from the Fisher Lake Pluton, Northern Sierra Nevada. Through a detailed study of the mineralogy and bulk chemistry of 70 individual layers, we show that comb layers and orbicule rims show no evidence of forming through a self-organizing, oscillatory crystallization process, but represent crystallization fronts resulting from in situ crystallization and extraction of evolved melt fractions during decompression-driven crystallization, forming a plagioclase-dominated cres-cumulate at the mm- to m-scale. We propose that the crystal content of the melt and the dynamics of the magmatic system control the mechanisms responsible for vertical igneous layering in shallow reservoirs. As comb layers crystallize on wall rocks, the higher thermal gradients will increase the diversity of comb layering, expressed by inefficient melt extraction, thereby forming amphibole comb layers and trapped apatite + quartz saturated evolved melt fractions. High-An plagioclase (An90–An97.5) is a widespread phase in Fisher lake comb layers and orbicule rims. We show that a combination of cooling rate, latent heat of crystallization and pressure variations may account for high-An plagioclase in shallow melt extraction zones.

The fluorine (F) and chlorine (Cl) contents of arc magmas have been used to track the composition of subducted components, and the F and Cl contents of MORB have been used to estimate the halogen content of depleted MORB mantle (DMM). Yet, the F and Cl budget of the Earth’s upper mantle and their distribution in peridotite minerals remain to be constrained. Here, we developed a method to measure low concentrations of halogens (≥0.4 µg/g F and ≥0.3 µg/g Cl) in minerals by secondary ion mass spectroscopy. We present a comprehensive study of F and Cl in co-existing natural olivine, orthopyroxene, clinopyroxene, and amphibole in seventeen samples from different tectonic settings. We support the hypothesis that F in olivine is controlled by melt polymerization, and that F in pyroxene is controlled by their Na and Al contents, with some effect of melt polymerization. We infer that Cl compatibility ranks as follows: amphibole > clinopyroxene > olivine ~ orthopyroxene, while F compatibility ranks as follows: amphibole > clinopyroxene > orthopyroxene ≥ olivine, depending on the tectonic context. In addition, we show that F, Cl, Be and B are correlated in pyroxenes and amphibole. F and Cl variations suggest that interaction with slab melts and fluids can significantly alter the halogen content of mantle minerals. In particular, F in oceanic peridotites is mostly hosted in pyroxenes, and proportionally increases in olivine in subduction-related peridotites. The mantle wedge is likely enriched in F compared to un-metasomatized mantle, while Cl is always low (

The Tonglvshan deposit is the largest Cu–Fe (Au) skarn deposit in the Edong district, which is located in the westernmost part of the Middle and Lower Yangtze River metallogenic belt, China. In this study, we performed a detailed in situ analysis of major and trace elements in amphiboles from the ore-related Tonglvshan quartz monzodiorite porphyry using electron microprobe (EMPA) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Two distinct populations of amphiboles, which can be distinguished by their aluminum content, are found in the quartz monzodiorite porphyry. The low-aluminum (Low-Al) amphiboles are subhedral or anhedral and formed at 46.3–73.5 MPa and 713–763 °C. In contrast, the high-aluminum (High-Al) amphiboles are euhedral and formed at 88–165 MPa and 778–854 °C. Some euhedral amphiboles are partially or completely replaced by Low-Al amphibole. The compositions of parental melts in equilibrium with the High-Al amphibole (Melt 1) and Low-Al amphibole (Melt 2) were computed by applying solid/liquid partition coefficients. This modeling shows that magma in equilibrium with High-Al amphibole (Melt 1) underwent 40% fractional crystallization of amphibole, plagioclase and apatite at a depth of ~5 km to evolve to magma in equilibrium with Low-Al amphibole (Melt 2). Copper enrichment occurred in the magma after undergoing fractional crystallization. The magma had a high oxygen fugacity, increasing from NNO + 1 (Melt 1) through NNO + 2 to HM (Melt 2), which could have prevented the loss of Cu (and possibly Au) to sulfide minerals during crystallization. Finally, the evolved magma intruded to shallower depths, where it presumably exsolved aqueous ore-forming fluids. Therefore, the large Cu–Fe–Au reserves of the Tonglvshan deposit can likely be attributed to a combination of controlling factors, including high oxygen fugacity, fractional crystallization, fluid exsolution, and a shallow emplacement depth.

Dacites dominate the large-volume, explosive eruptions in magmatic arcs, and compositionally similar granodiorites and tonalites constitute the bulk of convergent margin batholiths. Shallow, pre-eruptive storage conditions are well known for many dacitic arc magmas through melt inclusions, Fe–Ti oxides, and experiments, but their potential origins deeper in the crust are not well determined. Accordingly, we report experimental results identifying the P–T–H2O conditions under which hydrous dacitic liquid may segregate from hornblende (hbl)-gabbroic sources either during crystallization–differentiation or partial melting. Two compositions were investigated: (1) MSH–Yn−1 dacite (SiO2: 65 wt%) from Mount St. Helens’ voluminous Yn tephra and (2) MSH–Yn−1 + 10% cpx to force saturation with cpx and map a portion of the cpx + melt = hbl peritectic reaction boundary. H2O-undersaturated (3, 6, and 9 wt% H2O) piston cylinder experiments were conducted at pressures, temperatures, and fO2 appropriate for the middle to lower arc crust (400, 700, and 900 MPa, 825–1100 °C, and the Re–ReO2 buffer ≈ Ni–NiO + 2). Results for MSH–Yn−1 indicate near-liquidus equilibrium with a cpx-free hbl-gabbro residue (hbl, plg, magnetite, ± opx, and ilmeno-hematite) with 6–7 wt% dissolved H2O, 925 °C, and 700–900 MPa. Opx disappears down-temperature consistent with the reaction opx + melt = hbl. Cpx-added phase relations are similar in that once ~10% cpx crystallizes, multiple saturation is attained with cpx, hbl, and plg, +/− opx, at 6–7 wt% dissolved H2O, 940 °C, and 700–900 MPa. Plg–hbl–cpx saturated liquids diverge from plg–hbl–opx saturated liquids, consistent with the MSH–Yn−1 dacite marking a liquid composition along a peritectic distributary reaction boundary where hbl appears down-temperature as opx + cpx are consumed. The abundance of saturating phases along this distributary peritectic (liquid + hbl + opx + cpx + plg + oxides) reduces the variance, so liquids are restricted to dacite–granodiorite–tonalite compositions. Higher-K dacites than the Yn would also saturate with biotite, further limiting their compositional diversity. Theoretical evaluation of the energetics of peritectic melting of pargasitic amphiboles indicates that melting and crystallization of amphibole occur abruptly, proximal to amphibole’s high-temperature stability limit, which causes the system to dwell thermally under the conditions that produce dacitic compositions. This process may account for the compositional homogeneity of dacites, granodiorites, and tonalites in arc settings, but their relative mobility compared to rhyolitic/granitic liquids likely accounts for their greater abundance.

Amphibole has been discussed to potentially represent an important phase during early chemical evolution of arc magmas, but is not commonly observed in eruptive arc rocks. Here, we present an in-depth study of metastable calcic amphibole megacrysts in basaltic andesites of Merapi volcano, Indonesia. Radiogenic Sr and Nd isotope compositions of the amphibole megacrysts overlap with the host rock range, indicating that they represent antecrysts to the host magmas rather than xenocrysts. Amphibole-based barometry suggests that the megacrysts crystallised at pressures of >500 MPa, i.e., in the mid- to lower crust beneath Merapi. Rare-earth element concentrations, in turn, require the absence of magmatic garnet in the Merapi feeding system and, therefore, place an uppermost limit for the pressure of amphibole crystallisation at ca. 800 MPa. The host magmas of the megacrysts seem to have fractionated significant amounts of amphibole and/or clinopyroxene, because of their low Dy/Yb ratios relative to the estimated compositions of the parent magmas to the megacrysts. The megacrysts’ parent magmas at depth may thus have evolved by amphibole fractionation, in line with apparently coupled variations of trace element ratios in the megacrysts, such as e.g., decreasing Zr/Hf with Dy/Yb. Moreover, the Th/U ratios of the amphibole megacrysts decrease with increasing Dy/Yb and are lower than Th/U ratios in the basaltic andesite host rocks. Uranium in the megacrysts’ parent magmas, therefore, may have occurred predominantly in the tetravalent state, suggesting that magmatic fO2 in the Merapi plumbing system increased from below the FMQ buffer in the mid-to-lower crust to 0.6–2.2 log units above it in the near surface environment. In addition, some of the amphibole megacrysts experienced dehydrogenation (H2 loss) and/or dehydration (H2O loss), as recorded by their variable H2O contents and D/H and Fe3+/Fe2+ ratios, and the release of these volatile species into the shallow plumbing system may facilitate Merapi’s often erratic eruptive behaviour.

Differentiation of mantle-derived, hydrous, basaltic magmas is a fundamental process to produce evolved intermediate to SiO2-rich magmas that form the bulk of the middle to shallow continental and island arc crust. This study reports the results of fractional crystallization experiments conducted in a piston cylinder apparatus at 0.7 GPa for hydrous, calc-alkaline to arc tholeiitic magmas. Fractional crystallization was approached by synthesis of starting materials representing the liquid composition of the previous, higher temperature experiment. Temperatures ranged from near-liquidus at 1,170 °C to near-solidus conditions at 700 °C. H2O contents varied from 3.0 to more than 10 wt%. The liquid line of descent covers the entire compositional range from olivine–tholeiite (1,170 °C) to high-silica rhyolite (700 °C) and evolves from metaluminous to peraluminous compositions. The following crystallization sequence has been established: olivine → clinopyroxene → plagioclase, spinel → orthopyroxene, amphibole, titanomagnetite → apatite → quartz, biotite. Anorthite-rich plagioclase and spinel are responsible for a marked increase in SiO2-content (from 51 to 53 wt%) at 1,040 °C. At lower temperatures, fractionation of amphibole, plagioclase and Fe–Ti oxide over a temperature interval of 280 °C drives the SiO2 content continuously from 53 to 78 wt%. Largest crystallization steps were recorded around 1,040 °C and at 700 °C. About 40 % of ultramafic plutonic rocks have to crystallize to generate basaltic–andesitic liquids, and an additional 40 % of amphibole–gabbroic cumulate to produce granitic melts. Andesitic liquids with a liquidus temperature of 1,010 °C only crystallize 50 % over an 280 °C wide range to 730 °C implying that such liquids form mobile crystal mushes (

We conducted melting experiments on a low MgO (3.29 wt.%) basaltic andesite (54.63 wt.% SiO2) from Westdahl volcano, Alaska, at XH2O = 0.7–1 and fO2 ~ Ni–NiO, at pressures = 0.1–180 MPa and temperatures = 900–1,200 °C. We examine the evolution of the melt along a liquid line of descent during equilibrium crystallization at high H2O and fO2 conditions, starting from a high FeOt/MgO, low MgO basaltic andesite. Ti-magnetite formed on the liquidus regardless of XH2O, followed by clinopyroxene, plagioclase, amphibole, and orthopyroxene. We observe slight but significant differences in the phase stability curves between the XH2O = 1 and 0.7 experiments. Early crystallization of Ti-magnetite and suppression of plagioclase at higher pressures and temperatures resulted in strongly decreasing melt FeOt/MgO with increasing SiO2, consistent with a “calc-alkaline” compositional trend, in agreement with prior phase equilibria studies on basalt at similar H2O and fO2. Our study helps quantify the impact of small amounts of CO2 and high fO2 on the evolution of melts formed during crystallization of a low MgO basaltic andesite magma stored at mid- to shallow crustal conditions. Like the prior studies, we conclude that H2O strongly influences melt evolution trends, through stabilization of Ti-magnetite on the liquidus and suppression of plagioclase at high P–T conditions.

The crystallization sequence of a basaltic andesite from Bezymianny Volcano, Kamchatka, Russia, was simulated experimentally at 100 and 700 MPa at various water activities (aH2O) to investigate the compositional evolution of residual liquids. The temperature (T) range of the experiments was 950–1,150 °C, aH2O varied between 0.1 and 1, and the log of oxygen fugacity (fO2) varied between quartz–fayalite–magnetite (QFM) and QFM + 4.1. The comparison of the experimentally produced liquids and natural samples was used to constrain the pressure (P)–T–aH2O–fO2 conditions of the Bezymianny parental magma in the intra-crustal magma plumbing system. The phase equilibria constraints suggest that parental basaltic andesite magmas should contain ~2–2.5 wt% H2O; they can be stored in upper crustal levels at a depth of ~15 km, and at this depth they start to crystallize at ~1,110 °C. The subsequent chemical evolution of this parental magma most probably proceeded as decompressional crystallization occurred during magma ascent. The final depths at which crystallization products accumulated prior to eruption are not well constrained experimentally but should not be shallower than 3–4 km because amphibole is present in natural magmas (>150 MPa). Thus, the major volume of Bezymianny andesites was produced in a mid-crustal magma chamber as a result of decompressional crystallization of parental basaltic andesites, accompanied by mixing with silicic products from the earlier stages of magma fractionation. In addition, these processes are complicated by the release of volatiles due to magma degassing, which occurs at various stages during magma ascent.

The magmatic systems that give rise to voluminous crystal-poor rhyolite magma bodies can be considered to operate on two contrasting timescales: Those governed by longer-term processes by which a magma acquires its chemical and isotopic characteristics (e.g., fractional crystallisation and assimilation), and those operating at shorter timescales during the physical accumulation of the melt-dominant magma body that finally erupts. We explore the compositional and textural relationships between amphibole and orthopyroxene crystals from the 25.4 ka, 530 km3 (magma) Oruanui eruption products (Taupo volcano, New Zealand) to investigate how processes related to the physical assembly of the pre-eruptive magma body are represented in the crystal record. Over 90 % of orthopyroxenes from the volumetrically dominant high-SiO2 (>74 wt%) rhyolite pumices record textural evidence for a significant disequilibrium event (partial dissolution ± resorption of cores and interiors) prior to the growth of 40–500 μm thick rim zones. This dissolution/regrowth history of orthopyroxene is recorded in the chemistry of co-crystallising amphiboles as a prominent inflection in the concentrations of Mn and Zn, two elements notably enriched in orthopyroxene relative to amphibole. Textural and chemical features, linked with in situ thermobarometric estimates, indicate that a major decompression event preceded the formation of the melt-dominant body. The decompression event is inferred to represent the extraction of large volumes of melt plus crystals from the Oruanui crystal mush/source zone at pressures of 140–300 MPa (~6–12 km depth). Orthopyroxene underwent partial dissolution during ascent before reestablishing in the melt-dominant magma body at pressures of 90–140 MPa (~3.5–6 km). We model Fe–Mg diffusion across the core-rim boundaries along the crystallographic a or b-axes to constrain the timing of this decompression event, which marked establishment of the melt-dominant magma body. Maximum modelled ages indicate that this event did not begin until ~1,600 years before eruption, consistent with constraints from zircon model-age spectra. Once extraction began, it underwent runaway acceleration with a peak extraction age of ~230 years, followed by an apparent period of stasis of ~60 years prior to eruption. The rapidity of the extraction and accumulation processes implies the involvement of a dynamic driving force which, in the rifted continental arc setting of the Taupo Volcanic Zone, seems likely to be represented by magma-assisted extensional tectonic processes.

Accessory minerals contain a robust and accessible record of magma evolution. However, they may reflect relatively late-stage conditions in the history of the host magmas. In the normally zoned Criffell granitic pluton (Scotland), whole-rock (WR) compositions reflect open system assimilation and fractional crystallisation at depths of >11 km, whereas amphibole barometry and the absence of inherited zircon suggest that the observed mineral assemblages crystallised following emplacement of magmas with little or no crystal cargo at depths of 4–6 km. The crystallisation history is documented by large trace-element variations amongst apatite crystals from within individual samples: decreasing LREE and Th concentrations in apatite crystals from metaluminous samples reflect broadly synchronous crystallisation of allanite, whereas lower LREE and Th, and more negative Nd anomalies in apatites from peraluminous samples reflect the effects of monazite crystallisation. WR evolution is likely to have occurred within a deep crustal hot zone where H2O-rich (~6 wt%), low-viscosity magmas segregated and ascended adiabatically in a super-liquidus state, leading to resorption of most entrained crystals. Stalling, emplacement and crystallisation resulted from intersection with the H2O-saturated liquidus at ~4 km. H2O contents are as important as temperature in the development of super-liquidus magmas during ascent, blurring distinctions between apparently ‘hot’ and ‘cold’ granites. The trace-element contents of most accessory minerals are controlled by competitive crystallisation of other accessory minerals in small melt batches, consistent with the incremental assembly of large granitic plutons.

In volatile-saturated magmas, degassing and crystallisation are interrelated processes which influence the eruption style. Melt inclusions provide critical infor- mation on volatile and melt evolution, but this information can be compromised significantly by post-entrapment modification of the inclusions. We assess the reliability and significance of pyroxene-hosted melt inclusion analyses to document the volatile contents (particularly H2O) and evolution of intermediate arc magmas at Volcan de Coli- ma, Mexico. The melt inclusions have maximal H2O contents (B4 wt%) consistent with petrological estimates and the constraint that the magmas crystallised outside the amphibole stability field, demonstrating that pyroxene- hosted melt inclusions can preserve H2O contents close to their entrapment values even in effusive eruptions with low effusion rates (0.6 m 3 s -1 ). The absence of noticeable H2O loss in some of the inclusions requires post-entrapment diffusion coefficients (B1 9 10 -13 m 2 s -1 ) at least several order of magnitude smaller than experimentally determined H ? diffusion coefficient in pyroxenes. The H2O content distribution is, however, not uniform, and several peaks in the data, interpreted to result from diffusive H2O reequil- ibration, are observed around 1 and 0.2 wt%. H2O diffusive loss is also consistent with the manifest lack of correlations between H2O and CO2 or S contents. The absence of tex- tural evidence supporting post-entrapment H2O loss sug- gests that diffusion most likely occurred via melt channels prior to sealing of the inclusions, rather than through the host crystals. Good correlation between the melt inclusion sealing and volcano-tectonic seismic swarm depths further indicate that, taken as a whole, the melt inclusion popu- lation accurately records the pre-eruptive conditions of the magmatic system. Our data demonstrate that H2O diffusive loss is a second-order process and that pyroxene-hosted melt inclusions can effectively record the volatile contents and decompression-induced crystallisation paths of vapour- saturated magmas.

The pressure–temperature (P–T) conditions for producing adakite/tonalite–trondhjemite–granodiorite (TTG) magmas from lower crust compositions are still open to debate. We have carried out partial melting experiments of mafic lower crust in the piston-cylinder apparatus at 10–15 kbar and 800–1,050 °C to investigate the major and trace elements of melts and residual minerals and further constrain the P–T range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase + amphibole ± garnet) at 10–15 kbar and 800 °C, garnet granulite (plagioclase + amphibole + garnet + clinopyroxene + orthopyroxene) at 12.5 kbar and 900 °C, two-pyroxene granulite (plagioclase + clinopyroxene + orthopyroxene ± amphibole) at 10 kbar and 900 °C and 10–12.5 kbar and 1,000 °C, garnet pyroxenite (garnet + clinopyroxene ± amphibole) at 13.5–15 kbar and 900–1,000 °C, and pyroxenite (clinopyroxene + orthopyroxene) at 15 kbar and 1,050 °C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with 20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 10–40 wt% melting of lower crust composition at 10–12.5 kbar and 800–900 °C and 15 kbar and 800 °C closely resemble adakite/TTG rocks. TiO2 contents of the 1,000–1,050 °C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/TTG, partial melts produced at 10–12.5 kbar and 1,000 °C and 15 kbar and 1,050 °C have elevated HREE, whereas partial melts at 13.5–15 kbar and 900–1,000 °C in equilibrium with >20 wt% garnet have depressed Yb and elevated La/Yb and Gd/Yb. It is suggested that the most appropriate P–T conditions for producing adakite/TTG from mafic lower crust are 800–950 °C and 10–12.5 kbar (corresponding to a depth of 30–40 km), whereas a depth of >45–50 km is unfavorable. Consequently, an overthickened crust and eclogite residue are not necessarily required for producing adakite/TTG from lower crust. The lower crust delamination model, which has been embraced for intra-continental adakite/TTG formation, should be reappraised.

Migmatites are widespread in the North Dabie ultrahigh-pressure metamorphic terrane (NDT) of Dabie orogen, East China. Idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase, quartz and biotite, suggesting formation of leucosome by fluid-present melting of biotite + plagioclase + quartz-bearing protoliths at P = 5–7 kbar, T = 700–800 °C. Precise SIMS zircon U–Pb dating indicates that migmatization of Dabie orogen initiated at ~140 Ma and lasted for ~10 Ma, coeval with the formation of low-Mg# adakitic intrusions in Dabie orogen. Based on mineralogical, petrographic and geochemical data, leucosomes in NDT can be subdivided into three groups. (1) High La/Yb(N)–Medium Sr/Y group (Group I), whose high Dy/Yb(N) but medium Sr/Y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses. (2) Low La/Yb(N)–Low Sr/Y group (Group II), whose flat HREE patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss. (3) High La/Yb(N)–High Sr/Y and Eu# group (Group III), whose extremely high Sr and Eu but low other REE concentrations are caused by accumulation of plagioclase and quartz. Although Group I and III fall in the adakitic fields on La/Yb(N)–Yb(N) and Sr/Y–Y diagrams, they are chemically distinct from contemporary high-pressure adakitic intrusions in Dabie orogen in a series of geochemical indexes, for example, lower Dy/Yb(N) and/or Sr/Y ratios at given La/Yb(N) ratio, lower Sr/CaO ratios, lower Rb concentration but higher K/Rb ratios. Therefore, leucosomes are produced by anatexis of the exhumed ultrahigh-pressure metamorphic rocks at middle crustal level, instead of partial melting of thickened lower crust with garnet-rich and plagioclase-poor residual. The coeval occurrence of migmatites and high-pressure adakitic intrusions in Dabie orogen indicates large-scale partial melting of middle to thickened lower crustal column in the early Cretaceous. The required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed.

Presented here is a suite of new experiments aimed at quantifying the effects of pressure, temperature, bulk composition, and H[subscript 2]O content on the stability and composition of amphibole in the Earth’s mantle. Experiments have been performed from 2 to 4 GPa and 950 to 1100 °C on fertile and depleted mantle compositions. H[subscript 2]O contents of most experiments are 0.65 wt%. In the fertile mantle composition, pargasitic amphibole is stable up to ~3.8 GPa at 1000 °C, approximately 0.5 GPa higher than any previous study. The upper stability limit of amphibole in depleted mantle is 0.7 GPa and 40 °C lower than in fertile mantle. The addition of 3 wt% H[subscript 2]O to fertile mantle destabilizes amphibole by 0.5 GPa and 40 °C relative to the 0.65 wt% H[subscript 2]O experiments. Compared to existing experiments on amphibole stability, these experiments indicate that pargasitic amphibole may be stable in mantle lithosphere to almost 4 GPa (0.5 GPa higher (15 km deeper) than previously thought). The extremely strong destabilizing effect of H[subscript 2]O suggests that deeper portions of the strongly fluid-fluxed mantle wedge may be amphibole-free even at low temperatures near the slab–wedge interface. The molar alkali content of amphibole is shown to be a linear function (R[superscript 2] = 0.98) of pressure and temperature and is relatively insensitive to bulk compositional differences between fertile and depleted mantle. This relationship is used to produce an empirical thermobarometer for pargasite-bearing spinel and garnet lherzolites. Comparison to existing experimental data shows that this thermobarometer has predictive ability over the pressure range of 1–4 GPa. Comparisons with pressure–temperature estimates of garnet + amphibole peridotites further corroborate the applicability of this thermobarometer for natural samples. Pressure estimates are presented for four examples of metasomatized spinel peridotites otherwise lacking pressure information, and future avenues for refinement of the thermobarometer are discussed., National Science Foundation (U.S.) (Grants EAR-1118598 and EAR-1551321)

The aim of this study was to determine both ‘water’ contents (as OH− and H2O) and δD values of several clinopyroxene samples from alkaline basalts. These parameters were first obtained from five clinopyroxene samples using both the classical ‘off-line’ vacuum extraction technique and the ‘on-line’ high-temperature pyrolysis technique. Blanks measured with the ‘on-line’ gas extraction techniques were low enough to prevent any contamination by atmospheric water vapour. The comparison of data has revealed that our ‘on-line’ procedure is more effective for the extraction of ‘water’ from clinopyroxenes and, consequently, this ‘on-line’ technique was applied to ten additional clinopyroxene samples. Sample δD values cover a similar range from −95 to −45 ‰ (VSMOW) regardless of the studied locations, whereas the total ‘water’ content varies from ~115 to ~2570 ppm. The structural hydroxyl content of clinopyroxene samples measured by micro-FTIR spectrometry varies from ~0 to 476 ppm expressed in molecular water equivalent. The total ‘water’ concentrations determined by mass spectrometry differ considerably from structural hydroxyl contents constrained by micro-FTIR, thus indicating that considerable proportion of the ‘water’ may be present in (nano)-inclusions. The structural hydroxyl concentration—apart from clinopyroxenes separated from amphibole clinopyroxenite xenoliths—correlates positively with the δD values of clinopyroxene megacrysts for each locality, indicating that structurally bond hydrogen in clinopyroxenes may have δD values higher than molecular water in inclusions. This implies that there may be a significant hydrogen isotope fractionation for structural hydroxyl during crystallization of clinopyroxene, while for molecular water there may be no or only negligible isotope fractionation.

The halogen-bearing minerals tourmaline, amphibole, and biotite formed during magmatic–hydrothermal processes associated with the late-stage cooling of the Spirit Lake granitoid pluton (Mt. St. Helens, WA) and with the younger sulphide-mineralised rocks of the Margaret Cu–Mo porphyry deposit located entirely within the pluton. Major- and trace-element discrimination suggests that one tourmaline population crystallised from fractionated late-stage melt pockets in granodiorite–monzogranitic dykes of the pluton. These coarse, euhedral, oscillatory, and complexly sector-zoned uvite tourmalines span a limited range in Mg/(Mg + Fe) [Mg#] space (0.4–0.7 apfu) and show the highest Ti, Ca, F, Nb, and Ta contents, and low X-site vacancies ( 0.6 apfu), lower Ca and F contents, and the highest Li, As, and HREE contents (>80 ppm Li, >1200 ppm As). This population appears to record direct, rapid crystallisation from magmatic ± meteoric fluid(s) bearing the signature of the breakdown of primary feldspars and pyroxenes, with fluid exsolution from fractionated melt patches likely triggered by the formation of the previous generation of tourmaline. Mineralised porphyry deposit tourmaline compositions from the stockwork span a much larger range in Mg# space (0.05–0.9 apfu) and are almost entirely Ca-free. X-sites of these schorl tourmalines are dominated by Na or vacancies, and the Y-sites are strongly Fe enriched. The highest Mn and Zn concentrations (>4000 and >1000 ppm, respectively) potentially reflect the composition of mineralising fluids during ore deposition. A number of boron isotopic analyses yield predominantly heavy boron, but δ11B values range from −5.2 to 6.2 ‰ and average 1.4 ‰. Whilst most plutonic tourmalines conform to reported a- and c-sector element partitioning models, those from the mineralised porphyry show large and variable sector fractionation differences, suggesting that external controls may also be important. Wider evidence for late-stage pervasive metasomatism by halogen-bearing exsolved fluid(s) is provided by the high Mg# (>70) secondary amphiboles and biotites from within the Spirit Lake pluton, where the amphiboles are clear replacement products of primary pyroxenes. Fluid halogen fugacity ratios calculated from the biotite compositions overlap with other global mineralised porphyry systems, despite not being immediately associated with sulphide ores. The evidence suggests complex fluid processes and the coincidental development of the mineralised porphyry system within the pluton. Heat, fluids, and metals were therefore likely supplied by a later phase of magmatism, unrelated to the consolidation of the main Spirit Lake granitoid. These new constraints on magmatic–hydrothermal fluid signatures have wider applicability to potentially tracing proximal barren and mineralised processes, and for distinguishing between formation mechanisms for primary and secondary halogen-bearing minerals.

Phenocryst assemblages of lavas from the long-lived Aucanquilcha Volcanic Cluster (AVC) have been probed to assess pressure and temperature conditions of pre-eruptive arc magmas. Andesite to dacite lavas of the AVC erupted throughout an 11-million-year, arc magmatic cycle in the central Andes in northern Chile. Phases targeted for thermobarometry include amphibole, plagioclase, pyroxenes, and Fe–Ti oxides. Overall, crystallization is documented over 1–7.5 kbar (~25 km) of pressure and ~680–1,110 °C of temperature. Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 7.5 kbar for pyroxenes. Pyroxene temperatures are tightly clustered from ~1,000–1,100 °C, Fe–Ti oxide temperatures range from ~750–1,000 °C, and amphibole temperatures range from ~780–1,050 °C. Although slightly higher, these temperatures correspond well with previously published zircon temperatures ranging from ~670–900 °C. Two different Fe–Ti oxide thermometers (Andersen and Lindsley 1985; Ghiorso and Evans 2008) are compared and agree well. We also compare amphibole and amphibole–plagioclase thermobarometers (Ridolfi et al. 2010; Holland and Blundy 1994; Anderson and Smith 1995), the solutions from which do not agree well. In samples where we employ multiple thermometers, pyroxene temperature estimates are always highest, zircon temperature estimates are lowest, and Fe–Ti oxide and amphibole temperature estimates fall in between. Maximum Fe–Ti oxide and zircon temperatures are observed during the middle stage of AVC activity (~5–3 Ma), a time associated with increased eruption rates. Amphibole temperatures during this time are relatively restricted (~850–1,000 °C). The crystal record presented here offers a time-transgressive view of an evolving, multi-tiered subvolcanic reservoir. Some crystals in AVC lavas are likely to be true phenocrysts, but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crust.